1. Field of the Invention
The present invention relates to the fabrication of semiconductor components for the microelectronics industry and/or the optoelectronics industry. More specifically, it relates to the field of the fabrication and/or preparation of substrates intended for the production of such components.
2. Background of the Invention
There are certain substrate fabricating processes which consist in transferring a layer, for example a silicon layer, onto a support substrate.
A process for fabricating layers and thin films of semiconductor material, comprising at least three steps, is known, for example from document FR 2,681,472. During a first step, a layer of gaseous microbubbles is created by ion implantation under one face of a wafer of semiconductor material. During a second step, this face of the wafer is transferred onto a support substrate and fastened to the latter. During a third step, the assembly consisting of the wafer and the support substrate is subjected to a heat treatment capable of cleaving the wafer in the implantation layer. The choice of parameters, such as the time and the temperature of this heat treatment, depends on the history and the thermal budget of the semiconductor wafer. This thermal budget is, for example, acquired during the implantation step, by intentional heating and/or by heating due to the ion beam itself. It may also be acquired during an annealing step which precedes the bonding and which is intended to make the implanted atoms migrate for the purpose of facilitating the subsequent cleavage. It may furthermore be acquired during an annealing operation which is carried out before cleavage and is intended to stabilize the bonding. Other heat treatments may -also be envisaged before cleavage.
After cleavage, a thin layer adhering to the support substrate is obtained. This process is called the Smart-Cut(copyright) process.
In one particular application of this process, called the SOI (Silicon On Insulator) technique, a silicon-on-insulator layer is produced. Several ways may be envisaged for producing a silicon-on-insulator layer using the process described in the previous paragraph. According to a first way, it is possible, for example, to cover the silicon wafer, on its implantation face, with a layer of insulating oxide and to use a support substrate, for example also made of silicon, for the transfer. According to a second way, it is possible to have a completely semiconducting wafer which is transferred either onto a support substrate covered with a layer of insulator or onto a completely insulating support substrate (e.g. quartz) According to a third way, it is possible to have an insulator on the semiconductor wafer and to transfer this wafer either onto a support substrate which is itself also covered with an insulator or onto a completely insulating substrate. It should furthermore be pointed out here that, in order to obtain an insulator, it is advantageous to carry out a step of forming an oxide layer on the surface of a wafer or of a support substrate, in this case made of silicon, but more generally made of a semiconductor material.
After the three steps of the process described above, problems of the semiconductor layer disbanding from its support substrate may arise. Defects present at the interface between the semiconductor layer and the support substrate may also become electrically active and make the wafer composed of the support substrate/semiconductor layer assembly unusable. In order to alleviate these drawbacks, and more particularly to prevent the layer from disbanding when a polishing operation is envisaged, it is necessary to reinforce the bonding interface between the support substrate and the wafer having the semiconductor layer.
It is known that annealing at relatively high temperatures, i.e. greater than 1000xc2x0 C. and preferably about 1100xc2x0 C., allows the bonding interface to be reinforced. Hereafter, any thermal operation intended to improve the properties of the material we will call annealing. This annealing may be a heat treatment carried out at a constant temperature or at a varying temperature. In the latter case, the annealing may be carried out, for example, with a gradual increase in temperature between two values, with a cyclic oscillation between two temperatures, etc.
This type of annealing may be carried out in a non-oxidizing atmosphere or in an oxidizing atmosphere. Annealing in a non-oxidizing atmosphere (nitrogen, argon, vacuum, etc.) generally has the drawback of causing the spurious phenomenon of pitting on the surface of a semiconductor, particularly silicon. Annealing in an oxidizing atmosphere has the drawback of creating defects in the crystal structure. These defects are, for example, of the stacking-fault type and/or, in SOI structures, HF defects (a defect is called an HF defect when its presence is revealed by a halo of decoration of the buried oxide after treatment in a hydrofluoric acid bath), etc.
Moreover, it is sometimes useful, in the case of the application that we mentioned earlier for example, to form an oxide layer on the surface of a silicon layer, for example by oxidation. However, as indicated above, oxidation, but also more generally any formation of a surface oxide layer, is known to generate defects. Now, the presence of these defects in the crystal structure is thoroughly undesirable.
One object of the invention is to provide a process allowing annealing operations to be carried out, especially for stabilizing the bonding interface between a wafer comprising a semiconductor layer, especially a silicon wafer, and a support substrate, without any pitting of the surface of the layer.
Another object of the invention is to provide a process allowing an oxide layer to be formed on the surface of the semiconductor layer, while limiting as far as possible the number of defects introduced into the crystal structure.
These objects are achieved by virtue of a process for treating a substrate which includes a semiconductor layer on at least one of its faces, characterized in that it comprises a step of annealing the substrate and a step of forming an oxide layer on the surface of the semiconductor layer, which step is carried out before the end of the annealing step, protecting the rest of the semiconductor layer.
The expression xe2x80x9csubstrate which includes a semiconductor layer on at least one of its facesxe2x80x9d should be understood to mean an entirely semiconducting substrate (for example a silicon substrate), or a stack of semiconducting layers, or else a substrate comprising inhomogeneous structures or a substrate comprising components or parts of components at various stages in their fabrication.
By way of example, the semiconductor layers have a thickness of a few tens of xc3x85 to a few tens of microns.
Thus, by virtue of the process according to the invention, an oxide layer is formed on the surface of the semiconductor layer. This oxide layer protects the rest of the semiconductor layer, during the annealing step, in order especially to avoid the pitting phenomenon. The oxide layer may be formed by deposition of an oxide on the surface of the semiconductor layer (particularly, but not exclusively, in the case of nonoxidizable semiconductors), by thermal oxidation of the surface region of the semiconductor layer or else by deposition of an oxide on the surface of the semiconductor layer followed by thermal oxidation of the semiconductor through the oxide layer already deposited. In all cases, the oxide may be composed of elements in the semiconductor material and of other elements, such as nitrogen, etc.
The combination of the step of forming a surface oxide layer and of the annealing step, in the process according to the invention, makes it possible in particular to reinforce the bonding interface between the semiconductor layer and the support substrate, preventing the formation of defects, and more particularly the formation of pitting-type defects.
Moreover, the step of annealing the substrate makes it possible to heal the semiconductor layer of the defects generated during the previous steps in the fabrication and preparation process. More particularly, the annealing step may be carried out for a time and at a temperature which are such that the crystal defects, such as stacking faults, HF defects, etc., generated in the semiconductor layer during the step of forming a surface oxide layer are healed. Thus, it is possible to form an oxide layer on the surface of a semiconductor layer without dramatically increasing its level of defects. The Applicant has furthermore discovered that the healing of the semiconductor material by annealing gives it better resistance to any steps subsequent to the formation of an oxide layer on the surface of the semiconductor layer. This is because a semiconductor layer contains fewer defects after formation of a surface oxide layer, when it has been annealed prior to the formation of the oxide.
According to a variant of the process according to the invention, the process comprises, after the annealing step, a deoxidation step in order to remove the oxide layer formed on the surface of the semiconductor layer.
According to another variant, the process according to the invention comprises several steps of forming a surface oxide layer and several deoxidation steps, at least the final step of forming a surface oxide layer being followed by an annealing step.
According to the latter two variants, the process implemented according to the invention makes it possible in particular to thin the semiconductor layer, to remove part of the semiconductor layer containing a high concentration of defects or else to reduce the roughness of the surface of the layer. Thus, the process according to the invention proves to be particularly useful when, after the implantation, bonding and cleaving steps of the abovementioned process, it is desired, on the one hand, to remove the part disturbed by the implantation, i.e. in the cleavage zone (this part in fact contains an enormous number of defects) and, on the other hand, to reduce the roughness of the surface resulting from the cleavage. This formation of a sacrificial surface oxide layer on part of the semiconductor layer makes it possible to avoid the drawbacks of a polishing operation by itself. This is because the technique of polishing generates defects of the mechanical lesion type, strain-hardened zones, etc. When chemical-mechanical polishing is used, defects due to the chemistry may be added to the previous ones. In addition, the polishing generally results in lack of thickness uniformity. This latter drawback becomes more and more critical as the thickness of material to be removed increases, and therefore as the length of the polishing step increases. This is the case especially when the thickness to be removed by polishing reaches 100 nm. Thus, all these drawbacks usually result in a lack of reproducibility in the polishing results. In addition, lengthy polishing slow down the execution of the process and result in a drop in productivity. The advantages of forming a sacrificial surface oxide layer in accordance with the process according to the invention will therefore be appreciated, since it makes it possible to remove material and to thin a semiconductor layer. If this thinning is completed by formation of a sacrificial surface oxide layer in a polishing step, the defects generated by the polishing operation may then be developed on a smaller scale.
According to another variant, the support substrate covered with the semiconductor layer may be stored or delivered, for example to a semiconductor component fabricator, with a protective oxide layer which will be removed when the treatment of the substrate is continued.